Compression engine with direct fuel injection with compression ignition comprising means for cooling the piston

ABSTRACT

The present invention relates to a compression-ignition direct-injection internal-combustion engine comprising at least a cylinder ( 10 ), a cylinder head ( 12 ) carrying fuel injection means ( 14 ) projecting the fuel in at least two fuel jet sheets ( 36, 38 ) with different sheet angles (A 1,  A 2 ), a piston ( 16 ) sliding in this cylinder, a combustion chamber ( 34 ) delimited on one side by upper face ( 44 ) of the piston comprising a teat ( 48 ) extending in the direction of the cylinder head and arranged in the centre of a concave bowl ( 46 ) with at least two mixing zones (Z 1,  Z 2 ), and piston cooling means ( 76 ) housed in the material of the piston. 
     According to the invention, the cooling means comprise at least one circumferential gallery ( 78, 82 ) concentric to piston bowl ( 46 ) and arranged opposite at least one mixing zone (Z 1,  Z 2 ).

FIELD OF THE INVENTION

The present invention relates to a compression-ignition direct-injection combustion engine comprising piston cooling means.

It more particularly relates to such an engine usable in the air transport or road sector, or in the field of stationary equipments such as engine generators.

This type of engine generally comprises at least a cylinder, a piston provided with a teat arranged in a concave bowl and sliding in this cylinder in a reciprocating rectilinear motion, intake means for an oxidizer, burnt gas exhaust means, a combustion chamber and injection means for injecting a fuel into the combustion chamber.

As it is generally admitted, upon design of an engine, the performance, pollutant emission and mechanical strength constraints of the combustion chamber are increasingly high whereas the means for meeting them are quite the opposite.

Thus, performance increase generally leads to an increase in emissions and to higher mechanical stresses.

It is therefore necessary to overcome these stresses so as to guarantee limited pollutant emissions and satisfactory mechanical strength over the entire operating range of the engine, in particular at very high load. In particular for pollutant emissions, using all of the oxidizer present in the combustion chamber, for example an oxidizer comprising air at ambient pressure, supercharged air or a mixture of air (supercharged or not) and of recirculated burnt gas, is of great importance.

Indeed, the fuel mixture (oxidizer/fuel) in the combustion chamber needs to be as homogeneous as possible.

In practice, the fuel remains confined in the bowl and it cannot mix with the oxidizer contained notably in the squish area, i.e. in the volume located in the upper part of the combustion chamber delimited by the cylinder wall and the face of the cylinder head opposite the piston.

This involves the drawback of creating high richness areas in the combustion zone, generating a high production of soots, carbon oxide (CO) and unburnt hydrocarbons (HC) upon combustion of this fuel mixture.

Furthermore, to come back to the mechanical strength problem, the thermal load is focused on the re-entrant part of the piston, i.e. the bowl neck or diameter restriction that marks the transition between the piston bowl and the upper zone encompassing the squish area, which may be limiting in terms of mechanical strength at very high loads.

BACKGROUND OF THE INVENTION

To overcome these drawbacks, and as better described in French patent application No. 13/60,426 filed by the applicant, an internal-combustion engine comprising fuel injection means with jets having at least two sheet angles and a piston comprising a bowl provided with a teat with two combustion zone volumes and internal aerodynamics substantially improving the combustion quality is provided.

However, the will to increase the power density of this type of engine leads the designers to reconsider the size and therefore the mass of the various constituent parts of this type of engine.

It is noteworthy that the engine block is one of the most important parts of the engine and reducing the height of this part (deck height reduction) in order to significantly reduce the mass thereof is often considered. This height change notably leads to a piston height reduction.

Now, increasing the power density of the engine also involves increasing the specific power of the engine.

This causes a substantial increase in the mechanical stresses undergone by the constituent parts of the combustion chamber, such as the cylinder head, the cylinder and the piston. The will to maintain a high reliability level, notably regarding the mechanical strength for this type of engine, despite the necessary piston height reduction and the stress increase, requires using steel as the constituent material of this piston instead of the aluminium conventionally used, in order to enable better piston resistance.

However, steel has lower thermal conductivity than aluminium. It is thus necessary to use means allowing removal of the heat from the zones of this part most subjected to thermal stress, such as the re-entrant, the bowl bottom and the squish area.

As is widely known, notably through French patent application No. 2,839,116, it is possible to improve cooling of this type of piston using cooling means in form of an oil circulation gallery coupled with a specific oil spray system on the lower face of the piston.

However, such a gallery layout is not optimized to date, which causes unwanted piston temperature rises.

The present invention thus aims to overcome the aforementioned drawbacks by means of a piston that comprises galleries providing simple and inexpensive cooling thereof.

SUMMARY OF THE INVENTION

The invention therefore relates to a compression-ignition direct-injection internal-combustion engine comprising at least a cylinder, a cylinder head carrying fuel injection means projecting the fuel in at least two fuel jet sheets with different sheet angles, a piston sliding in this cylinder, a combustion chamber delimited on one side by the upper face of the piston comprising a teat extending in the direction of the cylinder head and arranged in the centre of a concave bowl with at least two mixing zones, and piston cooling means housed in the material of the piston, characterized in that the cooling means comprise at least one circumferential gallery concentric to the piston bowl and arranged opposite at least one mixing zone.

The shape of the section of the gallery can be cylindrical.

The cross-sectional area of the gallery can range between 20 mm² and 200 mm².

The surface area of one gallery can be different from the surface area of the other gallery.

The gallery can extend opposite the two mixing zones.

The distances between the bowl surfaces and the walls of the closest gallery can range between 2 mm and 7 mm.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying figures wherein:

FIG. 1 shows an internal-combustion engine according to the invention,

FIG. 2 is a large-scale partial view of a half section of the piston bowl profile of the engine of FIG. 1, and

FIG. 3 illustrates on a larger scale the layout of the cooling means in the piston of the engine of FIGS. 1 and 2.

DETAILED DESCRIPTION

With reference to FIG. 1, a compression-ignition direct-injection internal-combustion engine comprises at least a cylinder 10, a cylinder head 12 closing the cylinder in the upper part, fuel injection means 14 carried by the cylinder head and a piston 16, preferably made of steel, of axis XX sliding in the cylinder with a reciprocating rectilinear motion.

Fuel is understood to be a liquid fuel such as diesel fuel, kerosene or any other fuel with the physicochemical characteristics allowing operation of an engine of compression ignition type including a direct injection system for this fuel.

This engine also comprises a burnt gas exhaust means 18 with at least one exhaust pipe 20 whose opening can be controlled by any means such as an exhaust valve 22 for example, and an intake means 24 for an oxidizer with at least one intake pipe 26 whose opening can be controlled by any means such as an intake valve 28 for example.

An oxidizer is understood to be air at ambient pressure or supercharged air or a mixture of air (supercharged or not) and burnt gas.

The injection means comprise at least one fuel injector 30, preferably arranged along axis XX of the piston, whose nozzle 32 comprises a multiplicity of orifices through which the fuel is sprayed and projected in the direction of combustion chamber 34 of the engine.

It is from these injection means that the projected fuel forms at least two fuel jet sheets, here two sheets 36 and 38 of fuel jets 40 and 42, which, in the example shown, have a general axis merged with that of piston 16 while being axially positioned one above the other.

More precisely, sheet 36 that is the closer to piston 16 is referred to as lower sheet in the description hereafter, while sheet 38 that is further away from this piston is referred to as upper sheet.

As can be seen in FIG. 1, these two sheets form sheet angles A1 and A2 different from one another. A sheet angle is understood to be the top angle formed by the cone originating from the injector, whose imaginary peripheral wall passes through all the axes C1 or C2 of fuel jets 40 and 42.

Advantageously, sheet angle A1 of the lower sheet is at most equal to 130°, preferably ranging between 40° and 130°, while sheet angle A2 of the upper sheet is at most equal to 180°, preferably ranging between 150° and 180°.

For simplification reasons, in the rest of the description, angle a1 corresponds to A1/2 and angle a2 corresponds to A2/2 (see FIG. 2).

The difference between the two sheet angles is thus at least 50° so as to limit fuel jet overlaps between the two sheets and therefore formation of pollutants such as soots.

Of course, it is possible for the injection means not to be arranged along axis XX, but in this case the general axis of the fuel jet sheets from the fuel injector is at least substantially parallel to this axis XX.

Similarly, it is possible for each sheet to be carried by a distinct injector (single-sheet injector) with dedicated targeting in distinct zones of the combustion chamber.

Combustion chamber 34 is delimited by the inner face of cylinder head 12 opposite the piston, the circular inner wall of cylinder 10 and upper face 44 of piston 16.

This upper face of the piston comprises a concave bowl 46, whose axis is here merged with that of the cylinder, whose concavity is directed towards the cylinder head and which houses a teat 48 arranged substantially in the centre of the bowl, which rises towards cylinder head 12, while being preferably coaxial with the axis of the fuel sheets from injector 30.

Of course, it is possible for the axis of the bowl not to be coaxial with that of the cylinder, but the main thing is the layout according to which the axis of the fuel jet sheet, the axis of the teat and the axis of the bowl are preferably merged.

Furthermore, with reference to FIG. 2, teat 48, generally of truncated shape, comprises a preferably rounded top 50 extended, while moving symmetrically away from axis XX towards the outside of piston 16, by a substantially rectilinear inclined surface 52 extended by an inclined flank 54 down to a bottom 56 of the bowl.

Of course, without departing from the scope of the invention, inclined surface 52 can be nonexistent (zero length) and inclined flank 54 then connects the top of the teat to the bottom of the bowl.

In the example of FIG. 2, the bottom of this bowl is rounded, with a concave rounded surface 58 in form of an arc of a circle with radius R1, referred to as inner rounded surface, connected to the bottom of inclined flank 54, and another concave rounded surface 60 in form of an arc of a circle with radius R2, referred to as outer rounded surface, connected at one end to the lower end of the inner rounded surface at a point M and at the other end thereof to a lateral wall 62, substantially vertical here, at a point N.

The two rounded surfaces 58 and 60 thus delimit the lower part of a toroidal volume, here a torus of substantially cylindrical section 64 and of centre B whose purpose is described in the rest of the description.

Lateral wall 62 is extended, still while moving away from axis XX, by a convex rounded surface 66 in form of an arc of a circle with radius R3, referred to as re-entrant, leading to an inclined plane 68 linked to a concave inflection surface 69 connected to a substantially plane surface 70. This plane surface is continued by an outer convex surface 72 in form of an arc of a circle with radius R5 that leads to a plane surface 74 extending up to the vicinity of the cylinder wall.

The combustion chamber thus comprises two distinct zones Z1 and Z2 where mixing of the oxidizer they contain (air, supercharged or not, or mixture of air and recirculated burnt gas) with the fuel coming from the injector, as well as combustion of the fuel mixture thus formed, occurs.

For simplification reasons, these zones are referred to as combustion zones in the rest of the description.

Zone Z1, delimited by teat 48, torus 64 at the bowl bottom, wall 62 and convex rounded surface 66, forms the lower zone of the combustion chamber associated with lower sheet 36 of fuel jets of axis C1. Zone Z2, delimited by inclined plane 68, concave surface 69, substantially plane surface 70, convex surface 72, plane surface 74, the peripheral inner wall of the cylinder and cylinder head 12, forms the upper zone of this chamber associated with upper sheet 38 of fuel jets of axis C2.

In this configuration, the bowl comprises, for a piston position close to the top dead centre:

-   -   a bowl bottom outside diameter FD with a radius considered         between axis XX and the lowest point of the bowl, point M, i.e.         at the intersection between the surfaces of radius R1 and R2,     -   a bowl opening diameter BD with a radius considered in the         vicinity of the bowl bottom and corresponding to a distance         taken between axis XX and the furthest point of outer concave         surface 60,     -   a neck diameter GD with a radius corresponding to the distance         between axis XX and vertical wall 62 delimiting the outlet         section of this bowl,     -   an upper injection diameter 101 with a radius corresponding to         the distance between axis XX and the start of inflection surface         69 at point P between inclined plane 68 and concave surface 66         by delimiting a length L6 of jets 38 between the origin T2 of         axis C2 of the jets on the axis of the injector nozzle and point         P, meeting the formula ID1/sin(a2),     -   a developed length of the diametral half section Cb of the bowl,         consisting of the length from the intersection of the teat top         with axis XX to the cylinder wall,     -   a teat height H between the bowl bottom at point M and the top         of the teat,     -   a bowl height L between the bowl bottom at point M and plane         surface 74,     -   a junction height L3 corresponding to the extent of lateral wall         62 considered between the end of outer rounded surface 60 at         point N and the start of outer rounded surface 66,     -   a height L4 considered between point P and point M,     -   an angle of inclination a3 with respect to a vertical for         inclined flank 54,     -   an angle of inclination a4 formed by principal axis C1 of the         fuel jets of lower sheet 36 impacting the torus with the tangent         to impact point F by delimiting a length L5 of jets 40 between         origin T1 of axis C1 of the jets on the axis of the injector         nozzle and point F. This length L5 meets formula ID2/sin(a1),         with 102 corresponding to a lower injection diameter with a         radius that corresponds to the distance between axis XX and         point F,     -   an angle of inclination a5 considered at the tangency of outer         rounded surface 60 with lateral wall 62 at point N,     -   an angle of inclination a6 with respect to the horizontal and         the tangent to substantially plane wall 70,     -   an angle of inclination a7 with respect to the horizontal and         inclined plane 68 at intersection point P.

All these parameters are appreciated for a position of piston 16 in the vicinity of the top dead centre that corresponds to a distance D considered between point M and the origin T2 of axis C2 of jets 42.

More precisely, this distance D is equal to the sum of height L4 and height C, height C corresponding to the axial height between origin T2 and point P. This height corresponds to formula ID1/tan(a2).

Thus, the dimension and angle parameters of this bowl meet at least one of the following conditions:

-   -   angle a4 is greater than 80°, which amounts to passing over half         the fuel jet between centre B of torus 65 and the teat, and more         precisely the lower part at point M, and thus to generating an         aerodynamic upward motion in the torus towards the top of the         cylinder,     -   angle a5 must be positive and less than 90°. Preferably, it has         to be of the order of 30° to 40° so as to direct fuel jets 40 of         lower sheet 36 towards oxidizer volume S1 in order to use the         oxidizer of this zone while limiting upflow of this fuel towards         upper sheet 38,     -   oxidizer volume S1 between fuel jets 40 of the lower sheet is         minimized, still in order to optimize the use of oxidizer in the         chamber,     -   the position of the top of teat 48 is as close as possible to         nozzle 32 of injector 30 so as to limit the volume of oxidizer         under the injector that will not be impacted by the fuel jets,         which again amounts to minimizing volume S1. Thus, ratio H/L is         greater than 40% and preferably greater than 60%,     -   angle a3 is substantially equal to or greater than angle al of         the lower sheet) (−10°<a3−a1<10°). Thus, the general axis of the         lower sheet jets is tangential to flank 54 of the teat. Fuel         jets 40 of lower sheet 36 can thus interact with rounded surface         58 by vaporizing totally prior to impacting the piston,     -   oxidizer volume S2 between the two sheets is non-zero since the         interaction between the sheets is detrimental to the pollutants.         Volume S2 however needs to be minimized. Therefore, junction         length L3 between the torus and re-entrant 66 (convex rounded         surface of radius R3) has to be such that L3/(2*length of R2)<1         or (L3/length of R2<2) in order to ensure that oxidizer volume         S2 available between the upper 38 and lower 36 sheets is low in         relation to the fuel volume generated by the jets of the lower         sheet,     -   second combustion zone Z2 arranged in the upper part of the         piston that starts from re-entrant 66 is intended for fuel jets         42 of upper sheet 38,     -   the combustion volume of zone Z2 is at least equal to one tenth         of the total volume of the bowl,     -   the area referred to as squish area is formed by inclined plane         68, concave surface 69, plane surface 70, convex surface 72 and         plane surface 74. Angle a6 ranges between 10° and 75°, which         allows to burst fuel jets 42 so as to create an aerodynamic         motion above the piston and additionally to use the oxidizer in         the squish area. This aerodynamics allows better fuel/oxidizer         mixing above the piston, notably during expansion, thus         promoting oxidation of the burnt gas,     -   in order to promote the distribution of the fuel from jets 42 in         the squish area, a guiding surface 68 is provided between         re-entrant 66 and surface 70. This guiding surface can be         rounded in the continuation of the re-entrant or substantially         plane. The purpose of this guiding surface is to concentrate         fuel jets 42 and to guide them towards convex surface 72. Thus,         this guiding surface has an angle a7 at intersection point P         whose difference with respect to sheet angle a2 is less than         45°,     -   the position of inflection surface 69 is such that distances L5         and L6 are approximately of the same order (0.5<L5/L6<2). Thus,         advantageously, the fuel jets will impact substantially at the         same time the piston in the torus and the inflection zone         respectively,     -   diameter 101 must be such that ID1/GD>1 and         ID1<(GD+(Cb−GD)*2/3). This allows fuel jets 42 to optimize the         aerodynamics above the piston.

Furthermore,

-   -   ratio BD/L is less than 6, preferably less than 4,     -   ratio R2/R1 is less than 1, preferably less than 0.6,     -   ratio FD/BD is less than 1,     -   ratio Cb/BD is less than 2 so as to maintain complete         vaporization of the fuel and to prevent wetting of the cylinder         wall,     -   ratio GD/BD ranges between 0.7 and 1 for the torus aerodynamics         and upflow of the fuel jets,     -   ratio H/L is greater than 40%, preferably greater than 60% so as         to minimize the oxidizer volume between the injector nozzle and         the teat,     -   ratio L5/L6 ranges between 0.5 and 2 for the impact of the two         sheets at the same time,     -   A1 ranges between 40° and 130° with a1=A1/2,     -   A2 ranges between 130° and 180° with a2=A2/2,     -   a3 is substantially equal to a1,     -   a4 is greater than 80°,     -   a5 ranges between 0° and 90°, preferably substantially between         30° and 40°,     -   a6 ranges between 15° and 75°,     -   a7−a2 is less than 45°,     -   ratio ID1/GD is greater than 1,     -   ID1 is less than (GD+(Cb−GD)*2/3).

Thus, by means of this bowl parametrization, the fuel jets of lower sheet 36 directly target torus 64 and they do not directly impact re-entrant 66.

Therefore, combustion of the lower fuel/oxidizer mixture occurs essentially in the torus volume, whereas combustion of the upper fuel/oxidizer mixture occurs essentially in the squish area and above the piston.

Furthermore, the interaction of the upper sheet jets with the lower sheet jets is limited, which allows the fuel/oxidizer mixture to be homogenized while meeting mechanical strength constraints at high load.

The piston also comprises cooling means 76 including at least one gallery 78 associated with at least one combustion zone Z1, Z2.

This gallery 78 is a circumferential gallery concentric to axis XX, housed in the material of the piston, each one being arranged opposite at least one combustion zone Z1, Z2.

As is known per se, a cooling fluid such as engine lubricating oil is allowed into the gallery through an inlet line 80 substantially parallel to axis XX and it is discharged through a discharge line (not shown) of similar layout as the inlet line.

Thus, the cooling fluid circulates within the gallery while collecting part of the heat released upon combustion of the fuel mixture in the combustion zone, thus preventing an excessive piston temperature rise.

More specifically, gallery 78 allows to provide removal of the heat from parts of the bowl such as outer rounded surface 60, convex rounded surface 66 (re-entrant) and inclined plane 68 connected to concave inflection surface 69 linked with substantially plane surface 70 of combustion zone Z2.

Indeed, these parts are directly located where the combustion takes place and they therefore are the piston parts that undergo the highest thermal stress.

At least one circumferential cooling gallery 78 is thus provided in the material of the piston, close to the wall delimiting the bowl periphery including convex rounded surface 66 (re-entrant) and inclined plane 68 connected to concave inflection surface 69 linked with substantially plane surface 70. Advantageously, this gallery can also extend opposite zone Z1 in the vicinity of outer rounded surface 60.

Advantageously (see FIG. 1), at least another gallery 82 with its inlet line 84 and its discharge line (not shown) can be provided in the material of the piston, in the vicinity of outer rounded surface 60 of torus 64 of zone Z1 in such a way that gallery 78 does not extend as far as this surface.

These galleries 78 and 82 preferably have a cross-sectional area ranging between 20 mm² and 200 mm², and they are supplied with cold oil, by way of example, via oil injection or spraying into the gallery/galleries at piston bottom dead centre through inlet line(s) 80, 84. Gravity discharge of the oil occurs through the other line (not shown).

The cross-sectional area of the galleries, which may be different from one gallery to the other, thus allows to prevent formation of gas pockets detrimental to piston cooling and degradation of the mechanical qualities of the piston.

By way of example, the gallery can comprise a section of cylindrical shape, of substantially trapezoidal shape such as gallery 78 or of substantially circular shape as illustrated by gallery 82.

It is however essential to have sufficient piston material thickness between the combustion zone and the gallery so as not to affect the mechanical strength of the piston.

More precisely, in connection with FIG. 3 by way of example, the distance between the different surfaces of the bowl and the walls of the gallery closest to the bowl must range between 2 mm and 7 mm.

More precisely, distance E1, between inclined plane 68 connected to concave inflection surface 69 and upper wall 86 of gallery 78 closest to this inclined plane, distance E2 between outer rounded surface 60 and the wall of gallery 78, vertical wall 88 here, closest to this surface, and distance E3 between convex rounded surface 66 and convex surface 90 of this gallery must be contained in the aforementioned distance range.

Similarly, if the other gallery 82 is arranged opposite outer rounded surface 60, distance E4 between this rounded surface and the wall of the closest gallery must range between 2 mm and 7 mm.

The maximum 7 mm distance allows to provide sufficient cooling of the piston parts that undergo the highest thermal stress and thus to maintain a high reliability level. 

1. A compression-ignition direct-injection internal-combustion engine comprising at least a cylinder, a cylinder head carrying fuel injection means projecting the fuel in at least two fuel jet sheets with different sheet angles, a piston sliding in this cylinder, a combustion chamber delimited on one side by upper face of the piston comprising a teat extending in the direction of the cylinder head and arranged in the centre of a concave bowl with at least two mixing zones, and piston cooling means housed in the material of the piston, characterized in that the cooling means comprise at least one circumferential gallery concentric to piston bowl and arranged opposite at least one mixing zone
 2. An internal-combustion engine as claimed in claim 1, characterized in that the shape of the section of gallery is a cylindrical shape.
 3. An internal-combustion engine as claimed in claim 1, characterized in that the cross-sectional area of gallery ranges between 20 mm² and 200 mm².
 4. An internal-combustion engine as claimed in claim 1, characterized in that the surface area of one gallery is different from the surface area of the other gallery.
 5. An internal-combustion engine as claimed in claim 1, characterized in that gallery extends opposite the two mixing zones.
 6. An internal-combustion engine as claimed in any one of the claim 1, characterized in that distances between the bowl surfaces and walls of the closest gallery range between 2 mm and 7 mm. 